As requested by Etoile-sama, and hopefully of use to other costumers as well!

These are my Avatar contact lenses from 9mm SFX. They are 18mm scleral lenses, custom painted and custom made to my prescription (-10.0 and -8.0 correction). I adore them.

Comfort: The lenses are very large and very thick. You can definitely feel them in your eyes, especially right away. However, after a few minutes, they become less noticeable. After a full day of wearing them (16 hours), your eyes get tired and dry. Overall, I would rate them at an 8, if 10 = ordinary contact lenses.

Vision: My vision was a little restricted; you're looking out of a smaller space than normal. I found it less obvious in bright light and worse at night. I was able to function and navigate just fine. I might not want to drive wearing them, but I didn't try.

Customer Service: The folks at 9mm SFX were extremely helpful. I emailed them to ask about my prescription, which is higher than they usually sell. They had the lenses made up without any problems. Here's a timeline of the ordering process:
I ordered on March 1st. My eye doctor refused to confirm the prescription online, so I arranged with him and with 9mm SFX to have the clear scleral lenses shipped directly to the doctor. They arrived from the manufacturer (Coastal Contacts) one week later, on March 8. I went in to the doctor's office and had them fitted. The doctor asked me to wear them for a week and come back, just as he would for ordinary lenses. By the end of the fitting process, it was March 16th and I shipped the lenses back to 9mm SFX. By March 22, the online status said "payment received" and I assumed that meant they had received the lenses and thus charged my credit card. The final painted contacts arrived at my house on April 2nd.
So the total process took one month. That included manufacturing the lenses specifically to my prescription and the fitting process with my eye doctor. Your turnaround time may be faster.

Costume Effect: I love them. They really look alien and they add the perfect finishing touch to the Na'vi costume. They're less noticeable from a distance, but any close-up photo shows them off wonderfully. And even at a reasonable distance, you can tell my eyes are yellow. So yes, I would absolutely wear them again, for this costume and for any other yellow-eyed character (Desire from Sandman, for example).